My experience of unlimited holiday, several years in, is that some people don’t take enough and unfortunately some take far too much, almost every school holiday and all sorts, and managers find it hard to say no when it is unlimited. “An expectation of reasonableness” is open to challenge. You also have to have a “nominal” amount in mind for those on maternity leave and/or long term sick.
I find people prefer to have parameters to work within and it is fairer for all. Set a number, say 25 and people can buy more or sell 5. Add in a birthday day and a “me” day too. If you do go for it, I wouldn’t call it unlimited!
I agree with Karen on the sick pay. Keep it to 2 weeks a year for the coughs, colds, headaches and hangovers. Anything more is at manager discretion and should be for more serious illnesses or injuries.